1. Field of the Invention
The present invention relates to an optical deflection element which causes light rays having different wavelengths to exit in different directions, and more particularly to an optical deflection element which employs a photonic crystal.
2. Description of a Related Art
Heretofore, a passive element such as a prism or a diffraction grating has been employed as an optical deflection element. However, since the passive element such as a prism or a diffraction grating has a narrow deflection angle in causing light to exit therefrom, it requires a long optical path in order to satisfactorily split light rays having different wavelengths. Accordingly, in an apparatus which includes the optical deflection element for deflecting the light rays having different wavelengths in a wide range, it has been necessary to enlarge the element itself or to enlarge the whole apparatus for the purpose of gaining the long optical path.
Besides, in recent years, photonic crystals which exhibit unique dispersion characteristics as compared with conventional optical crystals have also been employed. The characteristics exhibited by the photonic crystals are stated in, for example, H. Kosaka et al. xe2x80x9cSuperprism phenomena in photonic crystalsxe2x80x9d, Physical Review B Vol. 58, No. 16, R10 096 to R10 099, 15 Oct. 1998-II), H. Kosaka et al. xe2x80x9cPhotonic crystals for micro lightwave circuits using wavelength-dependent angular beam steeringxe2x80x9d (Applied Physics Letters Vol. 74, No. 10, P. 1370-1372, 8 Mar. 1999, and Japanese Patent Application Laid-Open Publication JP-P2000-66002A.
Further, Japanese Patent Application Laid-Open Publication JP-P2000-56146A discloses a self-waveguide optical circuit in which photonic crystals are disposed at the main points of a substrate, whereby light is propagated in self-waveguiding fashion within the substrate so as to be split into a desired number of light rays. Furthermore, Japanese Patent Application Laid-Open Publication JP-A-11-271541 discloses a wavelength splitting circuit including two clads and a photonic crystal as its materials, for splitting light for every wavelength component owing to a structure in which the photonic crystal is interposed between the two clads.
The photonic crystal has been made smaller in size and higher in performance than the prism, the diffraction grating or the like by utilizing a superprism effect, but it still has a problem as stated below. Although the angle of light within the photonic crystal is greatly changed by a slight wavelength change, not only wavelength dispersion but also incident angle dispersion is great under the working conditions of the prior-art circuits disclosed in the above-mentioned patents, and hence, a high wavelength resolution cannot be attained considering the angular spread of an incident beam.
The present invention has been made in view of such a problem. An object of the present invention is to find out design conditions and working conditions for heightening a wavelength resolution and realize a high wavelength resolution in a wide wavelength range, in an optical deflection element which employs a photonic crystal.
In order to accomplish the object, an optical deflection element according to the first aspect of the present invention consists in an optical deflection element for splitting light rays in accordance with a wavelength of incident light, comprising a photonic crystal having an entrance end face and an exit end face, and having a refractive index which periodically changes depending upon positions thereof, wherein the element includes a region in which a propagation angle xcex8C of the incident light within the photonic crystal changes in accordance with the wavelength of the incident light having entered at an incident angle xcex8IN into the entrance end face so as to split the light rays having different wavelengths in accordance with different beam positions on the exit end face caused by a change of the propagation angle xcex8C based on the wavelength of the incident light, and a relationship among a wavelength xcex of the incident light in vacuum, a lattice constant xe2x80x9cAxe2x80x9d of the photonic crystal, the incident angle xcex8IN and the propagation angle xcex8C satisfies (∂xcex8C/∂(A/xcex))/(∂xcex8C/∂xcex8IN) greater than 10. Here, the element should preferably include the region in which the relationship (∂xcex8C/∂(A/xcex))/(∂xcex8C/∂xcex8IN) greater than 10 is met, in the whole wavelength range where the element is used, that is, in the whole predetermined wavelength range.
Further, an optical deflection element according to the second aspect of the present invention consists in an optical deflection element for splitting light rays in accordance with a wavelength of incident light, comprising a photonic crystal having an entrance end face and an exit end face, and having a refractive index which periodically changes depending upon positions thereof, wherein the element includes a region in which a propagation angle xcex8C of the incident light having a beam width 2w0 within the photonic crystal changes in accordance with the wavelength of the incident light having entered at an incident angle xcex8IN into the entrance end face so as to split the light rays having different wavelengths in accordance with different beam positions on the exit end face caused by a change of the propagation angle xcex8C based on the wavelength of the incident light, and a center wavelength xcex of a wavelength range of the incident light in vacuum, a lattice constant xe2x80x9cAxe2x80x9d of the photonic crystal, a refractive index n of a material at the entrance end face, a wavelength resolution xcex94xcex of the optical deflection element and an optical path length L along which the entered light propagates within the photonic crystal satisfy {(∂xcex8C/∂(A/xcex))/(∂xcex8C/∂xcex8IN)}xc2x72xcex3/(xcfx80nAw0) greater than xcex94xcex, and xcfx80nw02/{xcexxc2x7(∂xcex8C/∂xcex8IN)} less than L.
According to the present invention, it is possible to enhance the wavelength resolution of an optical deflection element employing a photonic crystal and to heighten the functions of the optical deflection element. Besides, since the size of the crystal necessary for obtaining a predetermined wavelength resolution can be calculated, the crystal of unnecessarily large size need not be prepared. It is accordingly permitted to reduce the size of the element and to curtail the cost thereof.